1. Field of the Invention
The present invention relates to the field of microwave engineering. The invention relates in particular to a coaxial connecting element for the microwave band and to a method for producing a connecting element such as this.
2. Description of Related Art
Coaxial components and connecting elements for the microwave band (from several GHz up to 250 GHz) generally have an outer conductor which extends along an axis and in which an inner conductor is arranged and held concentrically. One or more dielectric supporting element or elements is or are used for holding the inner conductor in the outer conductor. The configuration and the installation of the supporting elements influence not only the electrical but also the mechanical characteristics of the coaxial connecting element. On the one hand, the supporting elements should have as little influence as possible, in comparison with the air-cored line that is used as a reference, on wave propagation in the connecting element. On the other hand, the mechanical retention must be designed to be sufficiently permanent and precise that a large number of connecting cycles can be carried out with constant accuracy.
Examples of supporting elements such as these are disclosed, for example, in the documents U.S. Pat. No. 4,718,864, U.S. Pat. No. 4,867,703 or U.S. Pat. No. 5,269,702. The technology of hermetically sealed glass bushings is also known, in which an inner conductor is held by means of glass in a concentric metal ring, with the glass filling the annular space between the metal ring and the inner conductor, and being introduced by means of a melting process.
Another problem caused by the supporting elements is that higher transverse modes (in particular TE11) are stimulated in the area of the supporting elements, which leads to distortion of the transmitted signals and limits the range of operation of the connecting element in the direction of lower frequencies.
The article by R. Fuks, “New Dielectric Bead for Millimeter-wave Coaxial Components”, Microwave Journal No. 5, p. 318 ff (2001) explains this problem and, in order to solve it, proposes a novel type of supporting elements which is completely symmetrical on the radial plane and has inclined surfaces with respect to the axis. However, the supporting elements are produced as separate components and are then fitted, and this is at the expense of the precision of the arrangement.
Another way to hold the inner conductor in the outer conductor has been proposed in the document U.S. Pat. No. 4,456,324. In this known solution, the inner conductor is centred by and held by four curved dielectric platelets, which are held in a slotted hollow cylinder of the outer conductor and form the configuration of a Maltese cross, through whose centre the inner conductor passes. This type of retention has the advantage that only a very small amount of dielectric material need be used for support purposes. However, one considerable disadvantage is the complicated and complex assembly of the arrangement, which leads to considerable difficulties, especially in the case of coaxial arrangements with diameters in the millimetre range. Furthermore, the desired accuracies can be achieved only with difficulty because it is assembled from a large number of individual parts. The slots in the outer conductor are used to hold the dielectric platelets. No other function is recognized for them.